This invention relates generally to thermoplastic polymers and, in particular, to improving the tribological performance of thermoplastic polymers via thermally conductive media and other fillers and a process for making the same.
There has been a need for high performance reinforced plastic compositions having enhanced performance capabilities, from a standpoint of durability and longevity, when exposed to wear mechanisms encountered in a typical tribological environment. Such compositions generally have a unique combination of reinforcement materials incorporated into a plastic material.
Bearing materials comprised of self-lubricating composition material prepared from polymers have become popular in the friction and lubrication field because they are self-lubricating, rust-resistant, light in weight, easy to fabricate, relatively low in cost and very compatible. A large percentage of conventional metal bearing members have been gradually replaced by bearing members made from materials using polymers as a matrix.
An engine driveline is one example of a tribological environment where the use of plastic components for dynamic sealing and bearing applications is well known. In this environment, i.e., where a sealing or bearing interface is involved, the plastic component is exposed to friction, pressure, high temperature and lubricants. One such dynamic plastic component is a thrust washer, which is constantly subjected to a combination of varying levels of speed and load at high temperatures. Typically, a multiple of pressure and relative velocity (P*V) is used as a measure of how rigorous and demanding the application is. For example, a P*V value can range from as low as 50,000 to as high as 1,250,000, the pressure being measured in pounds/square-inch (p.s.i.) and the velocity being measured in feet/minute (f.p.m.). Applications having a P*V greater than 150,000 are generally considered to be very demanding.
When a thrust washer is used in a dynamic bearing application, it eventually fails either due to excessive wear at a given P*V, or high thermal stresses due to poor heat dissipation, e.g., xe2x80x9chot spottingxe2x80x9d, or sometimes a combination of both. Thus it is very desirable that the thrust washer has a high wear resistance at a given P*V so that it performs as a bearing, that it has good dissipation properties to avoid thermal stresses and that it has good flexibility to provide toughness and perform as a bearing. Thrust washers are typically made from plastics such as polyethersulphone (PES), polyamides (PA), polyaryletherketone (PAEK) and polyphenylenesulphides (PPS), to name a few. It would be advantageous to have a lower cost alternative to the PAEK-based thermoplastic bearing materials currently used in dynamic plastic components, including thrust washers.
Besides thrust washers, there are engine parts that are also exposed to tribological wear mechanisms. Sleeve bearings made from plastic compositions are constantly subjected to a harsh environment due to elevated temperatures encountered in the engine, as well as frictional wear and lubricants. Seal rings made from plastic compositions perform the dual function of a seal and a bearing and thus require a combination of high wear resistance and weld line strength without a significantly high flexural modulus.
It is desirable to have a thermoplastic composition that has excellent wear resistance and weld line strength properties without a significant increase in the flexural modulus. In addition, it would be desirable to have such a thermoplastic composition that is relatively insensitive to increases in load, has a medium for maintaining its temperature below its glass transition temperature, and exhibits excellent chemical resistance.
Furthermore, it is desirable to have a thermoplastic composition with the above qualities that is available at a lower price than comparable thermoplastic bearing materials currently on the market.
The present invention is directed to overcoming one or more of the problems set forth above.
In one aspect of this invention, a thermoplastic composition is disclosed. This thermoplastic composition includes a thermoplastic matrix that includes a resin and filler materials wherein the filler materials includes a combination of fibers, at least one lubricant, and thermally conductive material, for improving tribological performance of thermoplastic materials. In the alternative, a thermally conductive lubricant may be substituted for the combination of the lubricant and the thermally conductive material.
In another aspect of the present invention, a process for forming a product that functions in a tribological environment that includes the steps of compounding and molding a thermoplastic composition including a thermoplastic matrix that includes a resin and filler materials wherein the filler materials includes a combination of fibers, at least one lubricant, and thermally conductive material, for improved tribological performance of thermoplastic materials at lower cost. In an alternative, a thermally conductive lubricant may be substituted for the combination of the lubricant and the thermally conductive material.